1. Field of the Invention
This invention relates to a color display system in electrophoretic display devices.
2. Related Background Art
In recent years, with advancement of information machinery, the quantity of data of various information is becoming larger and larger, and the information is also outputted in various forms. The outputting of information is commonly roughly grouped into “display representation” making use of a cathode ray tube, a liquid crystal display panel or the like and “hard copy representation” on paper by means of a printer or the like.
In the display representation, there is an increasing need for display devices of low power consumption and small thickness. In particular, liquid crystal display devices have energetically been developed and commercialized as display devices adapted for such need. In liquid crystal display devices available at present, however, characters displayed on a screen may be viewed with difficulty depending on angles at which you look at the screen or under the influence of reflected light, and the task on eyesight which is caused by flickering, low luminance and so forth of a light source has not well been solved. Also, in the display representation making use of a cathode ray tube, although it provides sufficient contrast and luminance compared with the liquid crystal display, it may cause flickering for example, and can not be said to have a sufficient display quality level compared with the hard copy representation discussed below. In addition, its display units are so large and heavy as to have a very low portability.
Meanwhile, the hard copy representation has been considered to become unnecessary as information is made electronic, but in fact hard copies are still taken in a vast quantity. As reasons therefor, the following can be given. In the case of display representation of information (displayed on a screen), in addition to the above problem concerning the display quality level, the display has a resolution of 120 dpi at maximum, which is fairly lower than that of prints on paper (usually 300 dpi or higher). Hence, the display on a screen may greatly task eyesight compared with the hard copy representation. As the result, it often goes on that, even through the information can be seen on a display, it is first outputted on a hard copy. In addition, the information represented on hard copies can be arranged in a large number of sheets without any limitation of display area by the size of display as in the display representation, can be rearranged without any complicated machine operation, or can be checked in order. These are also large reasons why the hard copy representation is used in combination even though the display representation is feasible. Furthermore, the hard copy representation does not require any energy for retaining its representation, and has a superior portability that the information can be checked anytime and anywhere as long as the information is not so extremely much.
Thus, as long as any motion picture display or frequent rewriting is not required, the hard copy representation has various advantages different from the display representation, but has a disadvantage that paper is consumed in a large quantity. Accordingly, in recent years, development is energetically put forward on a rewritable recording medium (a medium on which highly visually recognizable images can repeatedly be recorded and erased in many cycles and which does not require any energy for retaining its representation). The third way of representation which has succeeded the features the hard copies have and in which images are rewritable is herein called “paper-like display”.
Requirements for the paper-like display are that images are rewritable, that any energy is not required or sufficiently a low energy is enough to retain the display (memory performance), that the display has a good portability, that the display has a good quality level, and so forth. At present, as a representation method which can be regarded as the paper-like display, for example a reversible display medium is available which makes use of an organic low molecular and high molecular resin matrix system which is recorded and erased with a thermal printer head (e.g., Japanese Patent Applications Laid-Open No. 55-154198 and No. 57-82086). This system is sometimes utilized as a display area of a prepaid card, but has problems such that the contrast is not so high and the writing and erasing can only be repeated a relatively small number of times, such as 150 to 500.
As a way of display which is expected to be utilized as another paper-like display, an electrophoretic display device invented by Harold D. Lees et. al. (U.S. Pat. No. 3,612,758) is known. Besides, Japanese Patent Application Laid-Open No. 9-185087 discloses an electrophoretic display device.
This display device is constituted of a dispersion medium having an insulating liquid in which colored electrophoretic particles stand dispersed, and a pair of substrates which are set face to face holding this dispersion medium between them. It is a device in which, upon application of a voltage to the dispersion medium via the electrodes, the colored electrophoretic particles are attracted by Coulomb force to the electrode side having polarity reverse to that of electric charges the particles themselves have, by utilizing electrophoretic properties of the colored electrophoretic particles. Its display is performed utilizing differences between the color of the colored electrophoretic particles and the color of an insulating liquid having been dyed. That is, the color of the colored electrophoretic particles is perceived when the colored electrophoretic particles are kept attracted to the surface of a first electrode near to the observer side and having light transmission properties. On the contrary, when the colored electrophoretic particles are kept attracted to the surface of a second electrode distant from the observer side, the color of the insulating liquid having been dyed is perceived, which has been so dyed as to have optical characteristics different from those of the colored electrophoretic particles.
However, in such an electrophoretic display device (hereinafter often “vertical movement type electrophoretic display device”), a coloring material such as a dye or ions must be mixed in the insulating liquid, and the presence of such a coloring material tends to act as an unstable factor in electrophoretic movement because it brings about the delivering and receiving of additional electric charges, resulting in a lowering of performance, lifetime and stability as a display device in some cases.
In order to solve such a problem, an electrophoretic display device in which an electrode pair consisting of a first display electrode and a second display electrode is disposed on the same substrate and the charged electrophoretic particles are made to move horizontally as viewed from the observer side has been proposed as disclosed in Japanese Patent Applications Laid-Open No. 49-5598 and No. 10-005727. It is a device in which, utilizing electrophoretic properties of colored electrophoretic particles, display is performed by making the colored electrophoretic particles move horizontally to the substrate surface between the surface of the first display electrode and the surface of the second display electrode in a transparent insulating liquid by applying a voltage.
In such a horizontal movement type electrophoretic display device, the insulating liquid is transparent in many cases. As viewed from the observer side, the first display electrode and the second display electrode are differently colored, and either of their colors has been made to have the same color as the colored electrophoretic particles. For example, where the color of the first display electrode is black, the color of the second display electrode is white and the color of the colored electrophoretic particles is black, the second display electrode comes uncovered to look white when the colored electrophoretic particles stand distributed over the first display electrode, and looks black as the color of the colored electrophoretic particles when the colored electrophoretic particles stand distributed over the second display electrode.
Now, the most fundamental system for materializing color display in the above electrophoretic display devices is a system in which three unit cells respectively having the three primary colors consisting of RGB or YMC are disposed in parallel on the same plane to make up each pixel and the color display is performed by the principle of additive mixture of color stimuli. In either system of the vertical movement type and the horizontal movement type, each unit cell has one kind of colored electrophoretic particles, two drive electrodes and a colored electrophoretic liquid, where two colors, the color of the colored electrophoretic particles and the color of the colored electrophoretic liquid, or the color of the colored electrophoretic particles and the color of a color filter, can be shown by the movement of the particles.
For example, in Japanese Patent Applications Laid-Open No. 2000-035589, three unit cells having different colored liquids with the three primary colors are disposed in parallel to form each pixel (FIGS. 21A to 21D). FIG. 21A shows a case of white display; FIG. 21B, a case of monochrome display; FIG. 21C, a case of complementary color display; and FIG. 21D, a case of black display. Unit cells formed of microcapsules in which a colored liquid and white particles have been enclosed are ejected from nozzles so that microcapsules having different colored liquids (electrophoretic liquids) with the three primary colors, yellow (Y), cyan (C) and magenta (M) are regularly arranged. Each microcapsule changes alternately in two colors, the white which is the color of the particles and the color of the electrophoretic liquid, by the vertical movement of the white particles.
In the case of the horizontal movement type also, three unit cells showing different colors for color display are similarly arranged to make up each pixel (FIGS. 22A to 22D). FIG. 22A shows a case of white display; FIG. 22B, a case of monochrome display; FIG. 22C, a case of complementary color display; and FIG. 22D, a case of black display. Each unit cell is filled with a transparent insulating liquid containing black particles. On the display electrode surfaces of the unit cells, different color filters with the three primary colors, red (R), green (G) and blue (B), are respectively disposed in order from the left cell. Each unit cell changes alternately in two colors, the black which is the color of the particles and the color of each color filter, by the horizontal movement of the black particles.
In International Publication No. 99/53373, a structure is disclosed in which unit cell microcapsules change in three colors. Three unit cells showing different colors for color display are arranged to make up each pixel. In this structure, which is called “dual particle curtain mode”, the unit cells are filled therein with an electrophoretic liquid in which two kinds of colored electrophoretic particles having different charge polarities and colors have been dispersed. By applying voltage to three drive electrodes, the two kinds of colored electrophoretic particles are made to move independently, where each unit cell can be made to change alternately in three colors, the colors of the two kinds of colored electrophoretic particles and the color of the electrophoretic liquid, or the colors of the two kinds of colored electrophoretic particles and the color of each color filter disposed on the back of each unit cell (FIGS. 23A to 23D). FIG. 23A shows a case of white display; FIG. 23B, a case of monochrome display; FIG. 23C, a case of complementary color display; and FIG. 23D, a case of black display.
In any of the above systems, when color display is performed, each pixel is formed by the three unit cells disposed adjoiningly and having colors corresponding to the three primary colors as shown in FIGS. 21A to 21D, FIGS. 22A to 22D and FIGS. 23A to 23D, and the desired display color is formed by the principle of additive mixture of color stimuli.
However, in the additive mixture of color stimuli of the three primary colors, it is theoretically impossible to achieve brightness and color sharpness (inclusive of sufficient black display) simultaneously, and it is very difficult to materialize a reflection type display device having the display quality level the printed mediums can have. Table 1 provides data showing, as indexes of display quality level in display devices, the ratios of reflected light intensity to incident light intensity in respect of white display, monochrome display (R, G, B), complementary color display (Y, M, C) and black display. It is considered that the white display intensity, the ratio of white display intensity to black display intensity and the absolute values of monochrome display intensity and complementary color display intensity reflect brightness, contrast and color sharpness, respectively. In Table 1, the numerals in parentheses in the columns of monochrome display and complementary color display are values including the white light component that does not contribute to color representation.
To regard with the conventional type display described above, in the case of the additive mixture of color stimuli by the use of white particles plus the three primary colors Y, C and M as in the structure shown in FIGS. 21A to 21D, a satisfactory level can be achieved in respect of the brightness, but colors of pastel shades lacking in color sharpness are shown because the white light component is always superimposed on the background of reflected light, and also any sufficient black is not obtainable. A sufficient black is obtainable if black particles are used, but such a measure is insufficient in respect of the brightness and the color sharpness.
On the other hand, in the case of the additive mixture of color stimuli by the use of black particles plus the three primary colors R, G and B as in the structure shown in FIGS. 22A to 22D, the intensity ratio of reflected light to incident light is 1/9 or less in the monochrome display and ⅓ or less in the white display, where any sufficient brightness is not achieved. The brightness is improved if white particles are used, but, like the case shown in FIGS. 22A to 22D, any sharp color representation is not obtainable and also any sufficient black is not obtainable.
In the structure shown in FIGS. 23A to 23D, the combination of colored electrophoretic particles with color filters which mutually stand a complementary color enables achievement of brightness about twice that of the structure shown in FIGS. 21A to 21D in respect of the monochrome display (FIG. 23B) and complementary color display (FIG. 23C), almost without damaging any color sharpness. There, however, is a problem that the reflected light intensity in the black display (FIG. 23D) is 33% and only performance with a contrast of about 3 is obtainable.
Meanwhile, a system in which plural kinds of particles having electrophoretic velocities different from one another and having different colors are dispersed in each pixel so that display colors can be changed by devising its driving method is proposed as disclosed in Japanese Patent Application Laid-Open No. 01-267525 (Toyota), U.S. Pat. No. 6,017,585, U.S. Pat. No. 6,067,185, U.S. Pat. No. 6,130,774, U.S. Pat. No. 6,172,798 (E Ink), Japanese Patent Application Laid-Open No. 2000-322007 (Brother) and so forth.
Each pixel is constituted of unit cells each containing i) two or three or more colored electrophoretic particles having different electrophoretic velocities (mobility, charge quantity and mirror force) and colors, ii) an electrophoretic liquid and iii) two drive electrodes, and the colors of a plurality of particles (plus the color of the liquid) are switched to materialize display in three or more colors. As also disclosed in Japanese Patent Applications Laid-Open No. 2000-194020 and 2000-194021 (Sony), different microcapsules are formed for every particles having a different electrophoretic velocity, and these are arranged in plurality to make up each pixel.
These systems have advantageous features that they make it unnecessary to area divide the pixels for each color, and enable bright display; and they make it unnecessary to independently provide the electrodes of a pixel for each unit cell, and can enjoy simple structure. However, their driving method in which particles arranged at display faces are separated and selected in a good precision only according to the magnitude of electrophoretic velocity is very difficult when gradational display is performed. It is also considered that the operation of writing which consists of a plurality of steps is not adaptable to the active matrix drive making use of thin film transistors (also called TFTs) and hence may bring a low utility.